Process for the obtention of purified heptafluoropropane

ABSTRACT

Process for the obtention of HFC-227ea having a reduced content of organic impurities, comprising at least subjecting a crude HPC-227ea to two distillation steps at different pressures.

The present invention concerns a process for the obtention of purified1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea).

HFC-227ea is useful in particular as environmentally-friendlyfire-extinguishing agent and as propellant, notably for pharmaceuticalaerosols.

The industrial production of HFC-227ea frequently provides, however, aproduct which comprises saturated and unsaturated impurities. As suchimpurities are often toxic, strict standards for the concentration ofimpurities, in particular unsaturated impurities are likely to beadopted for HFC-227ea for pharmaceutical applications.

In EP-0512502-A2 it was proposed to reduce or eliminate the olefin.content in HFC-227ea by reaction with an alcohol in the presence of abase. While this process provides outstanding results for theelimination of olefins in HFC-227ea, it requires addition of furtherchemical compounds to the crude HFC-227ea.

Accordingly, it is a purpose of the invention to provide a process forthe obtention of HFC-227ea having a reduced content of all kinds oforganic impurities and in particular olefinic impurities which does notrequire addition of further chemical compounds to the crude HFC-227ea.In particular, it is a purpose of the invention to provide such aprocess allowing for the obtention of HFC-227ea having a purity suitablefor use in pharmaceutical applications, especially as propellant forinhalation aerosols.

In consequence, the invention concerns a process for the obtention of1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea) having a reduced content oforganic impurities, which comprises

(a) subjecting a crude HFC-227ea containing organic impurities to atleast two distillations steps, consisting of a high pressuredistillation step and a low pressure distillation step carried out at apressure of at least 1 bar lower than the high pressure distillationstep and(b) recovering HFC 227-ea having a reduced content of organicimpurities.

The process according to the invention makes it possible to achieve anefficient separation of organic impurities, in particular of theolefinic impurities from HFC-227ea by a physical method. The process canbe carried out in an easy manner and allows for use ofsteam-heating/water-cooling. In a totally unexpected manner, the processis sufficiently efficient to allow for the use of the obtained HFC-227eain pharmaceutical applications. The process does not involve addition offurther chemical compounds to the crude HFC-227ea. This is particularlyadvantageous in the manufacture of HFC-227ea for pharmaceuticalapplications, as all compounds used in manufacture of a pharmaceuticalproduct must be examined as to their toxicity and notified in regulatoryprocedures.

The purity of the HFC-227ea obtained in the process according to theinvention is generally equal to or greater than 99.9% wt. Often thispurity is equal to or greater than 99.95% wt. Preferably, it is equal toor greater than 99.99% wt. The HFC-227ea obtained in the processpreferably contains less than 5 ppm of any individual olefinic impurity.

The term “low-boiling impurity” is understood to denote an impurityexhibiting, at the pressure of the distillation in the presence ofHFC-227ea, a boiling point lower than the boiling point of theHFC-227ea. The term “high-boiling impurity” is understood to denote animpurity exhibiting, at the pressure of the distillation in the presenceof HFC-227ea, a boiling point greater than the boiling point of theHFC-227ea.

The pressure in the high pressure distillation is generally at most 30bar. It is often at most 20 bar. Preferably, the pressure is at most 12bar. It is more preferably at most 11.5 bar. The pressure in the highpressure distillation is generally at least 6 bar. Often, this pressureis at least 7.5 bar. It is preferably at least 9.5 bar. Generally, afraction comprising low boiling impurities is drawn off from the highpressure distillation step. The low boiling impurities are therebyremoved from the HFC-227ea. Typical low-boiling organic impuritiesinclude saturated or insaturated fluorocarbons comprising 1, 2, 3 or 4carbon atoms. More particularly, low-boiling organic impurities includesaturated or unsaturated (hydro)fluorocarbons i.e. organic compoundsconsisting of carbon, fluorine and optionally hydrogen, comprising 1, 2or 3 carbon atoms. Hexafluoropropylene is a specific example of alow-boiling organic impurity.

The pressure in the low pressure distillation, is generally at least 1bar. It is often at least 4 bar. It is preferably at least 6 bar. It ismore preferably at least 7 bar. The pressure in the low pressuredistillation is at generally at most 12 bar. Preferably this pressure isat most 10 bar. Generally, a fraction comprising high boiling impuritiesis drawn off from the low pressure distillation. The high boilingimpurities are thereby removed from the HFC-227ea. Typical high-boilingorganic impurities include saturated or unsaturated (hydro)fluorocarbonscomprising at least 5 carbon atoms and (hydro)chlorofluorocarbonscomprising preferably at least two carbon atoms. More particularly,high-boiling organic impurities include saturated or unsaturated(hydro)fluorocarbons i.e. organic compounds consisting of carbon,fluorine and optionally hydrogen, comprising at least 6 carbon atoms.Hexafluoropropylene dimers of general formula C6F12 are specificexamples of high-boiling organic impurities.

Another type of high-boiling organic impurities includes organiccompounds comprising at least one heteroatom, which is preferablyselected from oxygen and nitrogen. Generally, these compounds alsocontain at least one fluorine atom. Mention may be made, for example offluorinated ethers comprising from 2 to 10 carbon atoms.

In the present description, any reference to the pressure corresponds tothe absolute pressure, measured at the top of the distillation column.

The pressure difference between the high pressure distillation step andthe low pressure distillation step is at least 1 bar. Preferably, thisdifference is at least 2 bar. A pressure difference of about 2.5 bar isparticularly preferred. Generally, the pressure difference between thehigh pressure distillation step and the low pressure distillation stepis at most 29 bar. Preferably, this difference is at most 10 bar. Apressure difference of at most 5 bar is particularly preferred.

The temperature at which the high or the low pressure distillation iscarried out corresponds approximately to the boiling point of theHFC-227ea at the pressure chosen for the respective distillation.

In a first embodiment of the process according to the invention the lowpressure distillation step is carried out before the high pressuredistillation step.

In a second embodiment of the process according to the invention, whichis preferred, the low pressure distillation step is carried out afterthe high pressure distillation step. In the second embodiment it ispossible to recover the HFC-227ea having a reduced content of organicimpurities from the top of the low pressure distillation. This mode ofrecovery is particularly advantageous, as it allows to minimise and evencompletely eliminate the risk of contamination of the HFC-227ea byoptionally present corrosion products of column material.

Each of the two distillation steps can be carried out in one or moredistillation columns. Use will preferably be made of a single column perdistillation step.

The distillation columns which can be used in the process according tothe invention are known per se. Use may be made, for example, ofconventional plate columns or plate columns of dual-flow type oralternatively of columns with bulk or structured packing.

The number of theoretical plates in the high pressure distillation isgenerally at least 10. It is often at least 20. A number of at least 35gives good results.

The number of theoretical plates in the low pressure distillation isgenerally at least 5. It is often at least 15. A number of at least 30gives good results.

The mass reboiling ratio in the high pressure distillation is generallyat least 1. Frequently, the mass reboiling ratio is at least 3. Morefrequently, the mass reboiling ratio is at least 8. A mass reboilingratio of at least 10 is preferred. The mass reboiling ratio in the highpressure distillation is generally at most 100. Frequently, the massreboiling ratio is at most 50. More frequently, the mass reboiling ratiois at most 30. A mass reboiling ratio of at most 25 is preferred.

The mass reflux ratio in the low pressure distillation is generally atleast 2. Frequently, the mass reflux ratio is at least 4. A mass refluxratio of at least 5 is preferred. The mass reflux ratio in the lowpressure column is generally at most 50. Frequently, the mass refluxratio is at most 30. A mass reflux ratio of at most 20 is preferred.

In a particular embodiment, the mass ratio in the high pressuredistillation of the feed of crude HFC-227ea to the fraction comprisingsaid low-boiling organic impurities withdrawn from said high pressuredistillation is kept in a range from 1 to 100. Preferably this massratio is kept in a range from 3 to 60. In a particularly preferredembodiment the mass ratio is controlled so as to display a deviationfrom the desired mass ratio of at most 10%. Preferably this deviation isat most 5%. Most preferably the deviation is at most 3%.

The high and low pressure distillations can be operated in continuous ordiscontinuous mode. In a preferred embodiment, the fraction comprisinglow-boiling organic impurities is continuously withdrawn from the highpressure distillation. The latter embodiment is used in a particularlyadvantageous manner, when the crude HFC-227ea contains a low amount oflow-boiling organic impurities.

Typical examples of crude HFC-227ea which can be used in the processaccording to the invention, have a content of low-boiling organicimpurities of 1% by weight or less. Often this content is 0.5% by weightor less. Preferably, this content is 0.3% by weight or less.

Typical examples of crude HFC-227ea which can be used in the processaccording to the invention have a purity of at least 95% by weight.Often the purity is greater than or equal to 99% by weight. Preferably,the purity is greater than or equal to 99.5% by weight.

The crude HFC-227ea can be obtained for example, throughhydrofluorination of hexafluoropropylene, through hydrofluorination ofsuitable halofluoropropanes or through hydrogenation of2-chloroheptafluoropropane. Advantageously, the crude HFC-227ea has beenobtained by a reaction comprising use of hexafluoropropylene as astarting material. Preferably, the crude HFC-227ea has been obtained byreaction of hexafluoropropylene with hydrogen fluoride, preferably inthe presence of a hydrofluorination catalyst.

If necessary, the crude HFC-227ea product obtained in reactions such asmentioned herebefore, may be worked-up by operations such as washing andrough-distillation to achieve the preferred purities for the crudeHFC-227ea which is introduced into the process according to theinvention.

In a still further embodiment, the crude HFC-227ea also comprises water.In this embodiment of the process according to the invention, thefraction comprising low-boiling organic impurities, which can bewithdrawn from the high pressure distillation step, also comprises waterwhich is thereby removed from the crude HFC-227ea. It should beunderstood that the specific preferences and general descriptions of theprocess according to the invention specifically apply to the presentfurther embodiment. It has been surprisingly found that water, whichnormally should be expected to be a high boiler with regard to HFC-227eaat all distillation pressures, forms an azeotrope with low boilingorganic impurities and HFC-227ea, thus enabling for efficient waterremoval from HFC-227ea. Water removal is especially advantageous when aHFC-227ea suitable for pharmaceutical aerosols is desired, as thepresence of water might influence in an undesirable way, the solubilityand crystal growth of medicaments to be dissolved or suspended in theHFC-227ea-comprising propellant.

Basically, the thermodynamic state of a fluid is defined by fourinterdependent variables: the pressure (P), the temperature (T), thecomposition of the liquid phase (X) and the composition of the gas phase(Y). A true azeotrope is a specific system of 2 or more components forwhich, at a given temperature and a given pressure, the composition ofthe liquid phase X is exactly equal to the composition of the gas phaseY. A pseudo-azeotrope is a system of 2 or more components for which, ata given temperature and a given pressure, X is substantially equal to Y.In practice, this means that the constituents of such azeotropic andpseudo-azeotropic systems cannot be readily separated by distillation.

For the purposes of the present invention, the expression“pseudo-axeotropic mixture” means a mixture of two constituents whoseboiling point (at a given pressure) differs from the boiling point ofthe true azeotrope by a maximum of 0.5° C. Mixtures whose boiling pointdiffers from the boiling point of the true azeotrope by a maximum of0.2° C. are preferred. Mixtures whose boiling point differs from theboiling point of the true azeotrope by a maximum of 0.1° C. areparticularly preferred.

The invention concerns also an azeotropic or pseudo-azeotropic mixturecomprising HFC-227ea and water. The azeotropic or pseudo-azeotropicmixture according to the invention consists generally, at a pressure of10.5 bar essentially of HFC-227ea and at most 500 mg/kg of water.Preferably the water content in the azeotropic or pseudo-azeotropicmixture at 10.5 bar is at most 300 mg/kg. A water content in theazeotropic or pseudo-azeotropic mixture at 10.5 bar of at most 200 mg/kgis particularly preferred. The azeotropic or pseudo-azeotropic mixtureaccording to the invention consists generally, at a pressure of 10.5 baressentially of HFC-227ea and at least 20 mg/kg of water. Preferably thewater content in the azeotropic or pseudo-azeotropic mixture at 10.5 baris at least 50 mg/kg. A water content in the azeotropic orpseudo-azeotropic mixture at 10.5 bar of at least 100 mg/kg isparticularly preferred.

The azeotropic or pseudo-azeotropic mixture according to the inventionconsists generally, at a pressure of 1 bar essentially of HFC-227ea andat most 50 mg/kg of water. Preferably the water content in theazeotropic or pseudo-azeotropic mixture at 1 bar is at most 30 mg/kg.The azeotropic or pseudoazeotropic mixture according to the inventionconsists generally, at a pressure of 1 bar essentially of HFC-227ea andat least 5 mg/kg of water. Preferably the water content in theazeotropic or pseudo-azeotropic mixture at 10.5 bar is at least 10mg/kg.

In a particular embodiment the azeotropic or pseudo-azeotropic mixtureaccording to the invention comprises in addition organic impuritieswhich are low-boiling with respect to HFC-227ea at the pressures of thehigh pressure distillation step such as described above.

The azeotropic or pseudo-azeotropic mixture according to the inventioncan be used to remove water and optionally low boiling organicimpurities from HFC-227ea.

The example given below is intended to illustrate, without impliedlimitation, the process according to the invention.

EXAMPLE

A crude HFC-227ea obtained by hydrofluorination of hexafluoropropenehaving a purity of about 99.8% wt. and containing 0.1% wt. low-boilingorganic impurities including hexafluoropropylene, 0.05% wt. high-boilingorganic impurities including C₆F₁₂ and water has been purified accordingto the process according to the invention. The crude HFC-227ea wasintroduced into the high pressure distillation step. The light-boilerswere continuously drawn off from the top of the high pressuredistillation. An HFC-227ea fraction containing high-boilers waswithdrawn from the bottom of the high pressure distillation andintroduced into the low pressure distillation. Purified HFC-227ea waswithdrawn from the top of the low pressure distillation. The tablehereafter indicates the process conditions which were used in the firstand low pressure distillation respectively.

High pressure Low pressure distillation distillation Pressure (bar) 10.5 8 Temperature (° C.) 55 45 Reboiling ratio (mass) 17.5 — Reflux ratio(mass) — 14 Theoretical plates 40 33 Mass ratio (feed crude 30 —HFC-227ea/low boiler fraction drawn off)

The obtained purified HFC-227ea had a purity of greater than 99.99% voldetermined by gas chromatography. The content of any olefinic impuritieswas less than 5ppm. The HFC-227ea contained less than 10 ppm of waterdetermined by Karl-Fischer method.

1.-14. (canceled)
 15. An azeotropic or pseudo-azeotropic mixturecomprising HFC-227ea and water.
 16. The azeotropic or pseudo-azeotropicmixture according to claim 15, at 10.5 bar consisting essentially ofHFC-227ea and water wherein the water content is at most 500 mg/kg. 17.The azeotropic or pseudo-azeotropic mixture according to claim 16,wherein the water content is at most 300 mg/kg.
 18. The azeotropic orpseudo-azeotropic mixture according to claim 17, wherein the watercontent is at most 200 mg/kg.
 19. The azeotropic or pseudo-azeotropicmixture according to claim 15, at 10.5 bar consisting essentially ofHFC-227ea and water wherein the water content is at least 20 mg/kg. 20.The azeotropic or pseudo-azeotropic mixture according to claim 19,wherein the water content is at least 50 mg/kg.
 21. The azeotropic orpseudo-azeotropic mixture according to claim 20, wherein the watercontent is at least 100 mg/kg.
 22. The azeotropic or pseudo-azeotropicmixture according to claim 15, at 1 bar consisting essentially ofHFC-227ea and water wherein the water content is at most 50 mg/kg. 23.The azeotropic or pseudo-azeotropic mixture according to claim 22,wherein the water content is at most 30 mg/kg.
 24. The azeotropic orpseudo-azeotropic mixture according to claim 15, at 1 bar consistingessentially of HFC-227ea and water wherein the water content is at least5 mg/kg.
 25. The azeotropic or pseudo-azeotropic mixture according toclaim 24, wherein the water content is at least 10 mg/kg.
 26. Theazeotropic or pseudo-azeotropic mixture according to claim 15,comprising in addition organic impurities which are low-boiling withrespect to HFC-227ea at a pressure of from more than 9 to 16 bar.